1. Field of the Invention
This invention relates to an automatic welding machine for arc-welding materials such as steel frames which have a straight welding line and form a welding groove.
2. Background Art
Recently, automating of arc welding is progressing. On the other hand, butt welding of straight parts or welding of multiple layers of materials forming a welding groove are relatively simple, and it has been considered that the automation of these operations can be readily achieved. However, in butt welding or in multilayer welding, the gap of the welding groove is seldom performed with high accuracy in the track welding of materials. This fact obstructs the automating of the welding operation.
A conventional automatic welding machine of this type is as shown in FIG. 11. FIGS. 12(a) and 12(b) are respectively a plan view and a front view showing one example of materials to be welded. In FIG. 12 are shown pieces 1 to be welded and a welding groove 2 between them. Further in FIG. 12(a), the welding direction is indicated by an arrow X, and the welding groove direction is indicated by an arrow Y.
In FIG. 11 is shown a block diagram of a control unit which is the center of the welding machine. It includes a control section 4 having a CPU (central processing unit) as its center and a memory section 5 for storing various data, the memory section 5 comprising a RAM (random access memory). Connected to the control section 4 are a welding condition setting section or terminal 6 made up of a keyboard and a data displaying unit and an input/output circuit 7 for transmitting data between the control section 4 and external equipment. A welding power source 8 is connected to the control section 4 through the input/output circuit 7, and supplies power to a welding head 9. The welding head 9 includes an X-axis drive motor 10 for moving the welding head 9 in the welding direction, a Y-axis drive motor 11 for moving the welding head 9 in the groove width direction, a potentiometer 12 whose phase changes with rotation of the Y-axis drive motor 11, and a welding torch 13.
Further in FIG. 11, a D/A (digital-to-analog) converter 14 converts a digital welding speed instruction value supplied by the control section to an analog value, which is given to the X-axis drive motor 10 through a motor drive circuit 15. A D/A converter 17 converts a digital oscillation pattern instruction value, supplied by the control section 4, to an analog value, which is supplied through amplifiers 18 and 19 to the Y-axis drive motor 11.
Fine control variable resistors 16 and 20 provide offset signals to respective adders 16a and 20a for the welding speed instruction value and the oscillation pattern instruction value, which have been subjected to digital-to-analog conversion. The offset signals may be derived from variable center taps of potentiometers, whose fixed end terminals are connected to positive and negative voltage sources. The input/output circuit 7 supplies a welding current value instruction to the welding power source 8 through a D/A converter 22 and it receives from the welding power source 8 an arc detection signal on a separate line.
The welding power source 8 is connected through a power cable 21 to the welding torch 13. The Y-axis drive motor 11 is so arranged as to be able to move the welding torch 13 in the width direction of the groove, while the X-axis drive motor 10 is so arranged as to be able to move the welding head 9 in the welding direction.
The operation of the machine thus organized will now be described.
Before starting a welding operation, the operator operates the terminal 6 to enter and set data for various conditions for the welding operation. The welding conditions are, for instance, a welding current, a welding speed, an oscillation width and an oscillation traversal time. The welding conditions are set by using the keyboard. The data, thus set, are applied through the control section 4 to the memory section 5, where they are stored in the RAM.
When a welding starting instruction is supplied to the control section 4 from the terminal 6, the control section 4 starts controlling the welding head 9 and the welding power source 8 according to the welding sequence.
The preset welding speed is read out of the memory section 5, and is applied through the input/output circuit 7 to the D/A converter 14, where it is converted into an analog value. The analog value is applied to the motor drive circuit 15, so that the X-axis drive motor 10 is driven to move the welding head 9 in the welding direction. The fine control variable resistor 16 is provided so that, during welding, the operator can adjust the welding speed with respect to the set value while observing the conditions of arcs. That is, the operator can suitably increase or decrease the welding speed.
In the case where the width of the welding groove is large to some extent, the welding torch 13 is oscillated in the groove width direction. For this purpose, an oscillation width, an oscillation traversal time, and both end stop times are set by using the welding terminal 6 similarly as in the case of setting a welding speed. The oscillation pattern is determined by the three conditions thus set.
The output instruction value of the oscillation pattern is calculated as position data after the control section 4 reads the three conditions from the memory section 5. The instruction value thus processed is outputted as position data for the Y-axis with respect to the time axis by the control section 4. The instruction value outputted by the control section 4 is supplied through the input/output circuit 7 to the D/A converter 17, where it is converted into an analog value.
The analog value is applied to the Y-axis drive motor after being amplified by the amplifiers 18 and 19.
On the other hand, the Y-axis drive motor 11 is coupled to the potentiometer 12, thus forming a conventional servo system. A difference signal representing the difference between the oscillation pattern output instruction value and the output voltage of the potentiometer 12 is produced by comparison and then amplified by amplifier 19. The signal thus processed drives the Y-axis drive motor 11 in such a manner as to eliminate the difference. The fine control variable resistor 20 is provided so that, during welding, the operator can finely adjust the oscillation width while observing the welding groove width and the arcing conditions. That is, the operator can suitably increase or decrease the oscillation width during welding.
The set welding current value, read out of the memory section 4, is applied through the input/output circuit 7 and the D/A converter 22 to the welding power source 8 to determine the welding current.
In the case where, as shown in FIG. 12, the welding groove width at the starting point of the welding is different from that at the ending point, it is necessary for the operator to adjust the oscillation width and the welding speed with the fine control variable resistors 16 and 20 at all times. And the fine adjustment is liable to include personal operator errors. Therefore, automatic control of the welding operation is not at all practical in this case.